This study will have two parts. In the first part (Observational Phase), patients receiving pegfilgrastim will be surveyed to document the incidence of significant pegfilgrastim-induced back and leg pain. In the second part (Treatment Phase), patients who previously experienced such pain will be randomized to receive a 7-day course of either the antihistamine loratadine or placebo to determine whether pegfilgrastim-induced back and leg pain can be prevented with this intervention.